The packaging industry has a high demand for transparent, highly glossy plastics films, for example biaxially oriented polypropylene or biaxially oriented polyester films. In addition, there is to an increasing degree a demand for those transparent films in which at least one surface layer is not highly glossy, but rather features a characteristic matt appearance and thus, for example, confers on the packaging a particularly attractive and therefore commercially effective appearance.
Typical applications for polyester films having at least one matt surface are outer packagings for coffee, tea, soups or of selected drugstore items. In addition to the commercially effective appearance, the matt side of the film renders the surface nonreflective.
EP-A-0 347 646 describes a biaxially oriented polyester film which has at least one overlayer A which contains a filler in a concentration of from 0.5 to 50%, in which the diameter of this filler is in a certain ratio to the layer thickness of the overlayer A. In addition, the overlayer A has a certain thickness and a certain degree of crystallization which is determined with the aid of Raman spectroscopy. As a consequence of the topography of the overlayer A, the film is especially suitable for magnetic recording tapes. The document gives no information about the gloss of the overlayer A achieved. A film produced according to EP-A-0 347 646 (Example 1) did not have the desired matt surface.
EP-A-0 053 498 describes a multilayer, biaxially oriented polyester film which has a transparent base layer (B) and a further layer (A) having a matt appearance and applied to at least one side of this layer. This layer having a matt appearance consists substantially of a polyethylene terephthalate copolyester which containsH(—OCH2CH2—)nOH orH(—OCH2—CH2—)n-1O—C6H4—O—(CH2—CH2O—)n-1H orH(—OCH2—CH2—)n-1O—C6H4—X—C6H4—O—(CH2—CH2O—)n-1H units(n is an integer from 2 to 140, X is —CH2—, —C(CH3)2— or —SO2—) and inert inorganic particles having an average diameter of from 0.3 to 20 μm in a concentration of from 3 to 40%, based on the layer having a matt appearance. The film features a high degree of mattness (gloss less than 15) and a transparency which is still acceptable for certain applications (greater than 60%). A disadvantage of this film is that it is not printable in the case of an ABA structure and cannot be processed, especially on high-speed machines, in the case of an AB structure. Moreover, it is too opaque for many applications and has deficiencies in the production.
The prior art likewise discloses matt, biaxially oriented polyester films having a milky appearance.
DE-A-23 53 347 describes a process for preparing a single-layer or multilayer, milky polyester film, which comprises forming a loosely blended mixture of particles of a linear polyester with from 3 to 27% by weight of a homopolymer or copolymer of ethylene or propylene, extruding the blend as a film, quenching and biaxially orienting the film by stretching it in mutually perpendicular directions, and heat setting the film. A disadvantage of the process is that the regrind (substantially a mixture of polyester raw material and ethylene or propylene copolymer) which occurs in the production of the film can no longer be reused in the film production, since the film otherwise becomes yellow. This makes the process uneconomic and the yellowish film produced with regrind was not able to become established on the market. When the concentration of the copolymer in the polyester is increased, the film generally loses its milky character and becomes white with high opacity.
U.S. Pat. No. 3,154,461 claims a biaxially oriented film of a thermoplastic (for example polyethylene terephthalate, polypropylene), said film having a matt surface and containing incompressible particles (for example calcium carbonate, silicon dioxide) in a size of from 0.3 to 20 μm and in a concentration of from 1 to 25%. However, this film is too opaque for many applications.
It is therefore an object of the present invention to provide a biaxially oriented polyester film which is matt on at least one side and does not have the disadvantages of the prior art films mentioned, and features in particular a high degree of mattness with simultaneously low opacity, in particular low volume opacity, and good transparency, very good producibility and very good processibility. In addition, it should be ensured that the film can also be processed on high-speed processing machines. In the production of the film, it should also be ensured that it is possible to recycle the offcut material occurring in the course of film production back to the production process as regrind, without the physical and optical properties of the film being significantly adversely affected.